The removal of copper and nickel from aqueous solution using Y zeolite ion exchangers

Colloids and Surfaces A: Physicochemical and Engineering Aspects 138 (1998) 11–20

The removal of copper and nickel from aqueous solution using Y zeolite ion exchangers Mark A. Keane * Department of Chemical Engineering, The University of Leeds, Leeds, LS2 9JT, UK Received 4 September 1996; accepted 23 February 1997

Abstract Nickel and copper removal from aqueous solution by batch ion exchange with solid lithium-, sodium-, potassium-, rubidium- and caesium-based Y zeolites have been studied under competitive and non-competitive conditions. The extent of transition metal (TM ) removal is dependent strongly on the nature of the out-going alkali metal (AM ) cation with the overall preference of the zeolite for exchange with both metals increasing in the order CsY
1. Introduction ‘‘Heavy metals’’ is a general collective term applied to the group of metals and metalloids with an atomic density greater than 6 g cm−3 and includes such elements as Cd, Cr, Cu, Hg, Ni, Pb and Zn which are commonly associated with pollution and toxicity problems. These pollutants reach the environment from a vast array of anthropogenic sources as well as natural geochemical processes [1]. Sources of copper and nickel pollutants include mining/smelting, agricultural materials, the electronics, chemical and metallurgical industries * Tel: +44 (0)113 2332428; Fax: +44 (0)113 2332405; e-mail: [email protected] 0927-7757/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0 9 2 7 -7 7 5 7 ( 9 7 ) 0 0 07 8 - 2

as well as waste disposal in the form of leachates from landfills [1–4]. Metal ions in solution adsorbed by aquatic plants and animals prove very toxic if the concentration is sufficiently high. Copper intake from drinking water can be around 1.4 mg per day from soft water and 0.05 mg per day from hard water [5], where the guideline for maximum acceptable copper concentration in drinking water is less than 3000 mg dm−3 [6,7]. Copper and nickel are among the most toxic metals for both higher plants and several microorganisms [8,9], whereas copper, along with arsenic and mercury, is recognized as exhibiting the highest relative mammalian toxicity [10]. The removal of heavy metal cations from aqueous solutions can be achieved by several processes,

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such as chemical precipitation, adsorption, solvent extraction, reverse osmosis, ultrafiltration or ion exchange [11–13]. Heavy-metal ion exchange from the liquid phase with benign ionic species in a solid phase is certainly an attractive option because of the basic simplicity of the application. Synthetic aluminosilicate zeolites act as efficient porous solid exchange media [14,15]. Zeolites possess ‘‘compensating’’ or charge-balancing cations which counterbalance the negative charge localized on the aluminosilicate framework and the exchange capacity is governed by the Si/Al ratio. As these cations are not rigidly fixed at specific locations within the hydrated unit cell it is possible to effect exchange with external cations in solution. Comparatively few papers dealing with transition metal ( TM ) ion exchange in zeolites have been published, and both nickel and copper ion exchange have essentially been viewed as a procedural step in the preparation of supported catalysts that have been successfully used to promote a wide range of reactions [16–19]. Nickel exchange with large pore zeolite Y (Si/Al=2.3) has been reported to be both temperature sensitive [20] and insensitive [21], whereas complete [21,22] and incomplete [23] removal of the parent sodium ions by divalent copper have been claimed. The overall selectivity of X and Y zeolites for bivalent TM ions has been shown by Maes and Cremers [21] to increase in the order Ni2+
not been properly assessed. The application of Y zeolites in removing nickel or copper or nickel/copper mixtures from aqueous solution is examined in this paper, where the influence of the zeolitic alkali metal (AM ) charge balancing cation (Li+, Na+, K+, Rb+ and Cs+), the heavy-metal to zeolite ratio and the exchange temperature are studied.

2. Experimental The starting zeolite was Linde molecular sieve LZ-52Y which has the unit cell composition Na (AlO ) (SiO ) (H O) . In order to 58 2 58 2 134 2 260 obtain, as far as possible, the as-received homoionic sodium form the zeolite was contacted five times with 1 mol dm−3 solutions of NaNO . The 3 zeolite was then washed briefly with deionized water, oven-dried at 363 K and stored over saturated NH Cl solutions at room temperature. 4 Maximally exchanged samples of LiY, KY, RbY and CsY were prepared by exhaustively exchanging 20 g aliquots of NaY (sieved in the mesh range 50–70 mm) with 200 cm3 portions of 0.1 mol dm−3 solutions of the appropriate nitrate in a 500 cm3 glass vessel fitted with a stirrer, condenser and thermocouple well. The resultant suspension was immersed in a thermostatically controlled oil bath (373±2 K ) and kept under constant agitation (600 rev min−1). The system was allowed to equilibrate for 48 h, at which point the zeolite was separated by filtration, washed briefly with hot deionized water (3×50 cm3), dried and equilibrated with water vapour as described above. Both the solid zeolite and the exchanging solution were analysed for all the exchanging ions. In the former case, the samples were prepared by treating 0.1 g of the hydrated zeolite with 20 cm3 conc. HCl and stirring at room temperature for 16 h. The resultant solution was then filtered and made up to 250 cm3 with deionized water. The lithium, sodium, potassium, rubidium and caesium contents were determined (to within ±2%) by flame photometry using a Perkin Elmer 360 spectrophotometer. The standards employed were prepared from stock solutions supplied by BDH Chemicals Ltd. The water contents of the various

M.A. Keane / Colloids Surfaces A: Physicochem. Eng. Aspects 138 (1998) 11–20

ion-exchanged samples were determined by thermogravimetry as described elsewhere [28]. In the case of the exchanging solution the filtrate was analysed as above, incorporating the necessary dilutions; the original 0.1 mol dm−3 exchanging solution was carried through as a blank. The partially exchanged zeolites were again submitted to the same exchange procedure, which was repeated until a constant level of exchange was attained. Zeolite crystallinity was measured by powder X-ray diffraction and IR spectroscopy [28]. The AM ion exchange of NaY proceeded to completion in the case of the lithium and potassium treatments, whereas rubidium and caesium exchange terminated at 69% of the total capacity of NaY. Nickel and/or copper removal from aqueous solution by ion exchange was conducted in the batch mode. The exchange isotherms (T=298 and 373±2 K ) were constructed at a total exchange solution concentration of 0.1 equiv. dm−3 where 1 equiv. equals 1 mol of positive or negative charge. The initial solution-phase TM to zeolite ratio ([ TM ] /zeolite, where s denotes the solution si phase and i initial values) was in the range (1–25)×10−2 mol dm−3 g−1 with exchange periods of up to 14 days. Isotherm points were obtained by contacting 0.2 g aliquots of the parent zeolite (the homoionic NaY or the maximally exchanged LiY, KY, RbY and CsY ) with 100 cm3 solutions containing either the salt of the in-going metal or the salts of both the in-going metal (binary exchange, Ni or Cu) or metals (ternary, Ni and Cu) and the indigenous ‘‘zeolitic’’ metal. The weakly acidic zeolite/salt solution suspension (pH minimum of 5.0) was kept under constant agitation at 600 rev min−1. After equilibration, the two phases were separated, the cation contents of both phases and the water content of the zeolite were measured, and zeolite crystallinity was routinely monitored by powder X-ray diffraction and IR spectroscopy as given above. The nickel and copper contents were determined by atomic absorption spectrophotometry to better than ±2% using an air–acetylene flame. All the chemicals employed in this study were AnalaR grade and were used without further purification.

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3. Results and discussion The chemical compositions of the maximally exchanged or parent Y zeolites are given in Table 1. The zeolitic water content decreased with increasing bare AM cation size; this may be attributed to the restrictions in the intracrystalline space available for water molecules. The differential thermograms of each of the maximally exchanged zeolites are characterized by a single peak where the temperature corresponding to maximum weight loss T varies from 441 K for LiY to 431 K for CsY; max the order of increasing T is in agreement with max increasing heats of hydration of the cations present. Linde Y zeolite is characterized by a ‘‘highsilica’’ open framework consisting of two independent, though interconnecting, three-dimensional networks of cavities, i.e. the accessible supercages of internal diameter 1.3 nm that are linked by shared rings of 12 tetrahedra (free diameter 0.7–0.8 nm) and the less accessible sodalite units that are linked through adjoining rings of six tetrahedra which form the hexagonal prisms (free diameter 0.20–0.25 nm). Complete removal of the indigenous sodium ion was achieved only in the case of the lithium and potassium exchanges. Rubidium and caesium exchange of NaY was found to terminate at 69% of the theoretical capacity, which agrees well with the literature values [29–31]. The larger rubidium and caesium cations cannot gain access through the hexagonal prisms to exchange with the sodium ions located within the sodalite units with the result that the maximally exchanged RbY and CsY contain 18 residual Na+ ions per unit cell. It can be stated that rubidium and caesium exchange in the Y zeolite matrix is limited to supercage sites. Sample crysTable 1 Unit cell compositions of the five hydrated parent Y zeolite ion exchangers Parent zeolite

Unit cell composition

LiY NaY KY RbY CsY

Li (AlO ) (SiO ) (H O) 58 2 58 2 134 2 263 Na (AlO ) (SiO ) (H O) 58 2 58 2 134 2 260 K (AlO ) (SiO ) (H O) 58 2 58 2 134 2 228 Rb Na (AlO ) (SiO ) (H O) 40 18 2 58 2 134 2 211 Cs Na (AlO ) (SiO ) (H O) 40 18 2 58 2 134 2 89

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tallinity, as monitored by X-ray diffraction and IR spectroscopy, was maintained for all the tabulated samples. The parent zeolite, when slurried in deionized water (10 cm3 g−1), produced a pH of 9 as the indigenous AM ions were partially exchanged with hydronium ions, where the free hydroxide ions that are formed are responsible for the alkaline pH of the slurries. Indeed, the calcined (in 120 cm3 min−1 dry air at 723 K for 18 h) parent Y zeolites exhibited ill-defined IR bands in the hydroxyl stretching region which can be attributed to some hydronium ion exchange. Nickel and copper exchange, on the other hand, was accompanied by significant decationation as a result of hydronium ion exchange, but the ratio of combined AM and TM ion content to the theoretical exchange capacity was ≥0.9. Hydronium ion exchange is particularly marked in the case of the copper system, as is illustrated in a characterization study of copper-exchanged Y zeolites that has been published previously [32]. Many TM salt solutions are sufficiently acidic to dealuminate or decompose the zeolite [33,34]. In this study, the X-ray diffractograms and IR spectra of representative nickeland copper-exchanged samples showed no significant deviation from those recorded for the parent zeolites. In addition to hydronium exchange, precipitation of TM hydroxides is known to occur [35,36 ]. TM salt imbibition in zeolites has, however, been considered negligible for external electrolyte concentrations <0.5 mol dm−3 [37]. The differential thermogravimetric traces for the higher nickel-loaded zeolites (exchange >60%) contained an ill-defined weight loss peak (<1% of total weight) at ca. 673 K which may be attributed to the presence of a hydroxylated nickel species that was deposited on the zeolite surface during ion exchange. A corresponding peak was not observed for copperexchanged samples, and the greater acidity of the copper salt solutions may act to neutralize partially the hydroxyl ions of the zeolite and inhibit hydroxide formation. However, the possibility of hydroxide deposition cannot be totally discounted. In short, AM ion exchange with external copper and nickel cations under the stated conditions is not totally stoichiometric due to competitive hydronium exchange and the possible occurrence of hydroxide formation in the solid.

3.1. Removal of nickel Exchange between the TM ion TM2+, initially in solution, and the AM ion AM+, initially in the zeolite, can be represented by the equilibrium TM2++2AM+=TM2++2AM+ (1) s z z s where the subscripts s and z refer to the ‘‘solution’’ and ‘‘zeolite’’ phases respectively. The exchange equilibrium for these ions can be characterized conveniently by the ion exchange isotherm, which is an equilibrium plot of the concentration of the exchanging ion in solution against the concentration of that same ion in the exchanger at constant temperature and constant charge concentration. The ion exchange isotherms for nickel exchange with the five parent Y zeolites at 298 K and 0.1 equiv. dm−3 are shown in Fig. 1, where the abscissa is the equilibrium nickel concentration in solution and the ordinate is the equilibrium concentration of nickel in the zeolite; subscript e denotes equilibrium values. It can be seen that the equilibrium distribution of nickel in the two phases is dependent on the nature of the out-going AM cation. Indeed, at equilibrium, zeolite-phase nickel concentrations less than 4×10−2 g g−1 the relaNi z tionship is linear for the five exchangers, as shown in the inset to Fig. 1. The slopes of these initial isotherm linear relationships provide some quantitative comparison of the influence of the AM cation in the exchange equilibria. The order of increasing gradient CsY (22 g g−1)
M.A. Keane / Colloids Surfaces A: Physicochem. Eng. Aspects 138 (1998) 11–20

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Fig. 1. Nickel exchange isotherms (T=298 K ) generated using LiY (+), NaY ($), KY (n), RbY (&) and CsY (,), where the equilibrium concentration of zeolite-phase nickel (grams of nickel per gram of zeolite) is plotted as a function of the equilibrium concentration of solution-phase nickel (grams of nickel per gram of solution). Inset: linear relationship between equilibrium zeolite and solution-phase nickel; symbols as above.

the stated conditions the exchange process is operating under diffusion limitations, where the progress of exchange is controlled by the diffusion of the ion within the crystal structure [14]. The effect of the aqueous environment on ion migration is pronounced, and in the aqueous exchange of zeoTable 2 Effect of varying the initial nickel solution concentration to zeolite weight ratio ([Ni ] /zeolite) on the concentration of si nickel removed from solution (D[Ni ]=[Ni ] −[Ni ] ) by each s si se AM parent zeolite 102[Ni ] /zeolite si (mol dm−3 g−1)

1.5 3.8 5.0 7.5 10.0 15.0 20.0 25.0

103D[Ni ] (mol dm−3) s LiY

NaY

KY

RbY

CsY

1.1 1.5 1.7 1.9 2.0 2.2 2.4 2.5

0.9 1.3 1.4 1.6 1.7 1.9 2.1 2.4

0.8 1.0 1.1 1.3 1.5 1.8 2.1 2.5

0.5 0.8 0.9 1.0 1.3 1.6 2.0 2.3

0.4 0.6 0.8 0.9 1.2 1.5 1.9 2.3

lite Y the migrating species are cation–water complexes where the cation in the zeolite phase is ‘‘solvated’’ to varying degrees by the lattice oxygen atoms. In the hydrated zeolite, the ions with the lower charge density, which are present in a less hydrated state, must interact more strongly with the aluminosilicate framework. Consequently, exchange of solution-phase nickel with the less hydrated or more framework-interacting Cs+ ions in the zeolite is retarded to a greater extent than is the case with more hydrated, less strongly interacting AM ions, particularly lithium and sodium. The sequence of zeolitic charge-balancing AM cations, Cs+, Rb+, K+, Na+ and Li+, can then be considered to represent a decreasing resistance to exchange with external cations with the result that at equilibrium the concentration of nickel in the zeolite phase correspondingly increases along this sequence. Nickel removal efficiency can be conveniently expressed as the quotient [Ni ] − si [Ni ] /[Ni ] and efficiency as a percentage is plotted se si as a function of the initial nickel to zeolite ratio

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in Fig. 2, taking the Ni/Li–Y and Ni/Cs–Y systems as representative cases. The greater effectiveness of the more hydrated lithium cation is again evident with nickel removal efficiencies in excess of 40% at initial nickel to LiY ratios less than 1×10−2 mol dm−3 g−1. A tenfold increase in this metal/exchanger ratio generates the same removal efficiencies regardless of the nature of the starting zeolite. Regeneration of the used zeolites is possible by back exchange with the parent AM ion in solution. Forward and reverse ion exchange data for the Ni/Na–Y system are presented in Fig. 3, where the forward direction (arrow left to right) refers to nickel removal from the liquid phase and the reverse direction (arrow right to left) refers to a regeneration of the solid Na–Y zeolite. The forward and reverse isotherms coincide to a greater degree and the exchange can be termed reversible. The nickel species removed from solution can, therefore, be released from the zeolite in a controlled fashion, and the zeolite exchanger is readily regenerated and is reusable. IR and X-ray diffraction analyses revealed no significant loss of zeolite crystallinity with repeated exchange cycles. 3.2. Removal of copper Equilibrium isotherm data for the removal of copper from aqueous solution are shown in Fig. 4.

Fig. 3. Equilibrium zeolite nickel concentration as a function of the initial nickel to zeolite ratio for exchange with NaY ($) and for the back exchange of a nickel-loaded zeolite ([Ni ] =6.9×10−2 g g−1) to regenerate the starting NaY ze Ni z exchanger (#).

As in the case of nickel exchange, the distribution of copper in the solid and liquid phases is dependent on the nature of the indigenous AM cation. Back exchange or regeneration of a maximally loaded Cu/Na–Y sample is illustrated in the inset to Fig. 4. Forward and reverse exchange data do not coincide to the same extent as observed for the nickel system. The (up to 20%) lower values of [Cu ] generated in the back-exchange step may ze be attributed to the added presence of hydronium ions incorporated into the zeolite framework during the forward exchange step, with the result that the Cu/Na–Y exchange system is not wholly reversible and the parent zeolite is not fully regenerated. The preference of the ion exchanger for one of two counter ions can be expressed by means of separation factors. The separation factor [Cu ] [AM ] ze se (2) [Cu ] [AM ] se ze is defined as the quotient of the concentration ratios of copper and the particular AM ion in the zeolite and in solution. If copper is preferred, the value of the separation coefficient is greater than unity; the converse holds if the AM ion is preferred. The relationship between the separation factor and the initial copper to zeolite ratio is illustrated by a=

Fig. 2. Exchange efficiency as a function of the initial nickel to zeolite ratio for LiY (+) and CsY (,).

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Fig. 4. Copper exchange isotherms (T=298 K ) generated using LiY (+), NaY ($), KY (n), RbY (&) and CsY (,) where the equilibrium concentration of zeolite-phase copper (grams of copper per gram of zeolite) is plotted as a function of the equilibrium concentration of solution-phase copper (grams of copper per gram of solution). Inset: equilibrium zeolite copper concentration as a function of the initial copper to zeolite ratio for the forward exchange with NaY ($) and for the back exchange (#) of a copperloaded zeolite ([Cu ] =8.4×10−2 g g−1). ze Cu z

means of logarithmic plots for the five parent Y zeolites in Fig. 5. From least squares analysis, the extrapolated values of a clearly illustrate the influ-

Fig. 5. Logarithmic plots illustrating the relationship between the separation factor and the initial copper to zeolite ratio for the removal of copper from aqueous solution using LiY (+), NaY ($), KY (n), RbY (&) and CsY (,).

ence of the nature of the AM ion. The order of increasing a at an initial copper to zeolite ratio of 1×10−5 mol dm−3 g−1 , CsY (680)0.22 mol dm−3 g−1) to CsY (>0.10 mol dm−3 g−1), again revealing the greater

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resistance to exchange exhibited by the less hydrated caesium ion. The efficiency of copper and nickel removal under identical experimental conditions is shown in Fig. 6 for the LiY, KY and CsY exchangers. In each instance copper is consistently removed with a greater degree of efficiency, but the difference is particularly marked at initial TM to zeolite ratios of <8×10−2 mol dm−3 g−1. It can be concluded that the solution-phase copper ions interact more effectively with the zeolite framework than the nickel ions which must remain strongly coordinated to the water molecules. Indeed, X-ray diffraction studies [38–40] have shown that the fraction of delocalized copper ions is appreciably lower than that of nickel in a fully hydrated zeolite. The Ni/Na–Y and Cu/Na–Y equilibrium exchange isotherms are both plotted in Fig. 7 for compara-

Fig. 6. Copper (open symbols) and nickel (closed symbols) exchange efficiencies as a function of the initial TM to zeolite ratio for (a) LiY, (b) KY and (c) CsY exchangers.

Fig. 7. Copper (&, solid line represents the polynomial [Cu ] =55.2[Cu ] −3.1×104[Cu ]2+6.5×106[Cu ]3) and nickel ze se se se (+, dashed line represents the polynomial [Ni ] =52.4 ze [Ni ] −2.8×104[Ni ]2+5.5×106[Ni ]3) exchange isotherms se se se (T=298 K ) generated using NaY, where the equilibrium concentration of zeolite-phase TM (grams of TM per gram of NaY ) is plotted as a function of the equilibrium concentration of solution-phase transition metal (grams of TM per gram of solution).

tive purposes. The isotherms were fitted to a thirdorder polynomial which adequately represents the observed relationship between the equilibrium zeolite and solution-phase TM concentrations; the polynomial coefficients are provided in the caption to Fig. 7. The copper system is characterized by a consistently higher equilibrium zeolite concentration. Both isotherms, however, exhibit a similar high affinity shape involving an initial steep slope followed by a gentle upward gradient, and at higher concentrations a sigmoidal relationship is discernible. Such a trend can be attributed to exchange heterogeneity involving an initial pronounced selectivity of the divalent ions for exchange with AM ions in the accessible supercages and an increasing involvement of exchange with parent ions located in the sodalite units at higher TM concentrations. The effect of exchange temperature on the separation factor is considered in Table 3. The higher values of a for the copper system are again diagnostic of the more efficient removal of copper. An increase in exchange temperature from 298 to 373 K was accompanied by an increase in the overall affinity

M.A. Keane / Colloids Surfaces A: Physicochem. Eng. Aspects 138 (1998) 11–20

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Table 3 The effect of exchange temperature on the separation factor a for nickel or copper removal by LiY at various TM/zeolite ratios 102[ TM ]/zeolite (mol dm−3 g−1)

a 298 K

1.0 2.5 5.0 7.5 10.0 15.0 20.0

373 K

Ni

Cu

Ni

Cu

13.9 7.1 4.3 3.6 2.5 1.4 0.7

25.4 11.8 6.0 4.6 3.5 2.3 1.3

56.7 13.0 6.7 5.7 4.0 2.4 1.2

78.2 19.3 7.8 6.4 4.9 3.7 2.0

of the zeolite phase for both TMs, as evidenced by the increase in the values of a. Such an increase in TM removal may be attributed to a weakening of the ion–dipole interactions between the in-going ions and the coordinated water molecules where the hydration sheath is stripped and the TM ions are more effectively solvated by the zeolite framework oxygen atoms. The progress of TM uptake by the solid is, therefore, dependent not only on the nature of the out-going AM ion, but also on the degree of hydration of the in-going TM ion.

Fig. 8. Effect of varying the initial TM (where TM represents Ni or Cu) solution concentration to KY zeolite weight ratio ([ TM ] /zeolite) on the concentration of nickel (TM=Ni, +) si or copper ( TM=Cu, $) removed from solution: D[ TM ]=[ TM ] −[ TM ] . s si se

3.3. Removal of both nickel and copper The higher efficiency for the removal of copper exhibited by each Y zeolite in single TM component solutions suggests that copper should be removed preferably from combined copper/nickel solutions. The concentration of both copper or nickel removed by KY from a range of ternary Ni2+/Cu2+/K+ solutions is plotted in Fig. 8 as a function of the initial TM (copper or nickel ) to zeolite ratio. The total charge concentration was maintained at 0.1 equiv. dm−3 but the individual TM concentrations were varied in the range 1×10−3–4.5×10−2 mol dm−3. The data were fitted to third-order polynomials in order to assist a visual assessment of the overall trend. Indeed, at the same initial concentrations, copper was removed from solution to an appreciably greater extent. Removal of both metals was, nevertheless,

Fig. 9. Linear relationship between the equilibrium and initial solution-phase copper to nickel ratios, the former generated after treatment with KY at 298 K.

increased with increasing initial concentrations. The scatter of data is due to varying amounts of potassium in solution which balance the overall positive charge at a particular copper/nickel concentration and influence to a degree the equilibrium distribution of both TMs. Regardless, the marked preference of the zeolite for exchange with copper over nickel under non-competitive condi-

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tions, as illustrated in Fig. 7 and Table 3, clearly extends to solutions containing both metals. A linear relationship (slope=0.76) was observed between the equilibrium and initial copper to nickel solution concentrations, as shown in Fig. 9. Such a relationship provides a quantitative measure of the effective TM removal under competitive conditions; in this study copper removal by KY at 298 K was 1.3 times that of nickel.

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